The FBI routinely uses software exploits to install something called CIPAV on the remote client computer to retrieve forensics data and negate the effect of proxies, vpns, tor, etc.

http://www.wired.com/threatlevel/2007/07/fbi-spyware-how/

Twitter almost assuredly has the ability to push a custom iframe or similar based on who is logged on to support these kinds of government payloads. In the case the wired article references myspace directly assisted.

Surely being more security paranoid than usual should make it harder for an attack like this to succeed, but if the feds get pissed enough it's not unthinkable that they could get access to a targeted zero day to use.